Technical Case Concentricity

[Mtn Grizzly]

My friend and I both just took delivery on 6.5 X 47 Lapua rifles and are in the process of load development. This ongoing development has raised a question that eludes us both. I started with 150 Lapua cases that were trimmed, weight sorted to <.001, Neck wall thickness was <.001. After one firing, case runout was <.0005 to flat. After sizing using a Harrells press, Redding type S Bushing die and Imperial lube - my cases had the following runout.
15 <.001
34 =.001
45 =.0015
40 =.002
16 =.003

Keeping the cases in lots (based on previous runout) I fired the cases a second time, re-sized them and checked their runout. Everything had changed! For instance-the lot that had 40 cases with .002 runout now had:

4 <.001
15 =.001
10 =.0015
7 =.002
4 =.003

QUESTION: Can anyone explain why concentricity changes (better and worse) from firing to firing on cases with known parameters? Thank you for your help Mtn Grizzly

 

[fdshuster]

MtnGrizzly: I also get some variation, with Lapua, (222,223,220 Russian/6ppc, 6BR & 308), checked on my Sinclair concentricity gauge. Your range of runout is about what I also am getting, and I don't consider it to be a problem. If and when it is more than .003", I mark the case(s) and use them for basic scope adjustment, first round foulers, and a maximum distance of 100 yds. What causes it? I don't know, and write it off as another one of those variables. Maybe someone else will add some thoughts. ???

 

[glo]

If you have not already chronographed all your loads, you might want to try that. Then see if case run-out is proportionate to measured velocities. I would suspect it to be. The case run-out you are experiencing is nothing more than the normal spring back of the brass under the varying camber pressures at or just above maximum chamber pressure.

Even if you are full length sizing die, the brass will tend to follow the same amount of spring back as it did under chamber pressure. You would not likely see this phenomenon if you are using custom full length sizing die made from the roughing reamer that cut your chamber. This too if you are using a small base sizing die.

 

[wapiti25]

Some of this relates as to articles by Craiten Audett about slecting cases with uniform walls in the body. Also how the brass responds to the sizing die.

 

[Mikem]

Mtn Grizzly,
The next time you check your brass for neck run-out, (Concentricity) take a magic marker and mark the Low side of the case neck. Fire that round, re-due all of your normal brass prep. Now check the neck run-out as before, and again mark the Low side, and note if the new mark is in the same place (or close to it) as the last time you marked it.
If so, check the neck wall thickness in relation to the Low mark, you may find the Low marked side will be the thick side of the neck,(or close to it) and this thickness may run all the way down the inside to the case head, effecting how the brass responds to sizing etc.
If you test like this, let us know what you find, I have been working this problem for some time.
Mike.

 

[Winchester69]

Neck sizing?

 

[Laurie]

I think most precision minded handloaders see this and wonder about the cause(s). I'd imagine any and all of other contributors' suggestions apply. Some people don't restrict wall measurements to the neck but do the body too claiming the neck alone can mislead as to case wall thickness consistency.

I suspect it's mainly the mix of tolerances and variations in the tools/components - dies + press + cases - and slight variations in operator technique from case to case. Either way, I don't worry about 0.001" runout and wouldn't segregate cases on that size of figure.

One thing I've noticed and seen confirmed by others is that some very good die sets such as Redding Competition neck-sizers seem to inject a degree of neck-runout no matter what you do, (often larger than one thou'), but the bullet seating die subsequently wipes it out so that bullet runout is less than case-neck runout, as opposed to runout getting worse as each action is performed which is what you'd expect and is the norm with many die sets. I can't explain it, but am just thankful it happens.

Laurie,
York, England

 

[Benjerman08]

Griz,
I had this same exact thing happen this week. Last week I loaded 100 long range shells and when I ran them over the concentricity gauge (bullets on loaded shells), I found the concentricity nothing less than fantastic. 75 of them were in the .001-.0015 range, The rest were in the .002 range, but only one over that. I will take that with unsorted brass any day. After reloading the same cases this week with the same components, I have only 50 really good ones, 30 in the .002-.003 range and 20 in the .004-.005 range. I clean my dies each time loading and had forgot to loosen the top of the neck sizing die which allows the bushing to float slightly which lines up the neck better. Looking back, I remembered that some of them seemed to size with a different feel. I believe the bushing was slightly off to one side. Just a thought from this end.

 

[watercam]

I second Laurie's post, I have a 6.5/284 whereby the case necks on loaded ammo runout around .002" while the bullet runout is pretty much zero. Gun shoots great. My 6XC has cases that run essentially zero as do the bullets. I can't explain it but I know the results. If the gun shoots well at long range, it's all good.
Mike

 

[Bast]

Too much shoulder bump may affect the run out. Measure it and don't bump over .002

 

[CPorter]

I have a hypothesis which may be total BS. When you fire a cartridge the firing pin pushes the cartridge forward into the chamber until the shoulder tops out and the firing pin strikes the primer anvil. The primer sets off the powder charge and pressure builds in the case and following the path of least resistance the case expands out at the thinnest parts until it presses against the chamber walls. When it can't grow radially because of the chamber walls it will start to expand axially from the neck towards the bolt. Parts of the chamber are rougher than other parts and keep that part pinned causing the section closer to the head to expand more. If those areas slightly exceed their plastic deformation limits they will spring back less causing a bit of run out. The area around the neck and top of the shoulder are a bit rougher due to the powder residue that squeezes by as the bullets leave and before the pressure seal those areas against the chamber. The thick brass around the web keeps it from expanding radially as fast as the section above it and it grows axially until the head contacts the bolt. This is where you get case head separation and run out on the base. Keeping the chamber smooth and polished helps the brass seal and grow uniformly along its whole length except for the area that powder residue affects. Neck sizing minimizes the case stretch and any inequalities in spring back. As the neck gets work hardened the powder line you see after firing works it way further towards the head. Keep brass annealed allows the neck to expand and seal the chamber efficiently and the powder line stays about midway down the neck.